This proposal describes a series of genetic and biochemical approaches that will be used to study the components of the sulfur-signal transduction cascade that is responsible for the acclimation responses of Chlamydomonas reinhardtii to sulfur deprivation. Initial work will generate a collection of insertional mutants that lack the normal responses to sulfur deprivation. Genetic analysis will be used to determine if the mutant phenotype segregates with the insertion. The genetic lesions in these mutants will be determined and complementation will be attempted by transforming the mutants with the wild-type locus. This information will be used to generate a model for the sulfate signal transduction cascade using a genetic approach. Using the pathway model, biochemical methods will investigate the possible interaction between components of an individual pathway, as well as to discover new interactors. The wild-type and mutant strains will be used for biochemical studies focused on determining which of the proteins involved in the acclimation of cells to sulfur deprivation physically interact. This will help build a more detailed picture of the signaling pathway.